This invention is related to a method and apparatus for processing seismic data to produce an image of the subsurface. More particularly, this invention is related to a method and apparatus for determining the source signature of a land vibrator which generates phase locked seismic energy into the subsurface such that regardless of whether the vibrator is phase locked to the baseplate, reaction mass or ground force, the correlation of the source signature and seismic data acquired by recording the reflections of the phase locked seismic energy will produce images of the subsurface which are substantially the same.
Numerous techniques for exploring the earth to acquire seismic data are well known. One well known technique utilizes a land vibrator for generating vertically-oriented seismic energy into the earth. A land vibrator transmits a frequency modulated signal into the medium being explored. The transmitted signal is reflected off subsurface reflecting interfaces and directed towards the surface for detection by geophones positioned on the surface. As the reflections so recorded are caused by a seismic source activated at a known time and location, the recorded data can be processed by a computer in known ways to produce an image of the subsurface. The image thus produced is commonly interpreted by geophysicists to detect the possible presence of valuable hydrocarbons.
A second well known technique also utilizes a land vibrator for generating vertically-oriented seismic energy into the earth. Here, the generated energy is reflected off subsurface reflecting interfaces and detected by a geophone located in a borehole. Preferably, the source should be placed such that the subsurface reflection points generally lie in the plane containing the borehole and the source location. The geophone would then be moved to a new location for a next shot with the distance between geophone locations being some constant distance such as 50 feet. See, for example, U.S. Pat. No. 4,627,036 issued to Wyatt and U.S. Pat. No. 4,597,464 issued to Chelminski.
In operating vibrator exploration systems, it is important to ensure that the energy imparted into the ground is in phase with the reference signal. Several different types of phase locking are known in the art----baseplate (or "BP") phase locking, reaction mass (or "RM") phase locking, ground force (or "GF") phase locking and ground force phase locking with amplitude control. Traditionally, baseplate phase locking whereby the signal generated by the baseplate of a vibratory seismic source imparting seismic energy into the ground is maintained in phase with the reference signal (also referred to as the "filtered pilot sweep" or "FPS") being supplied to the vibratory seismic source, has been selected for use in vibratory exploration systems. For example, see U.S. Pat. No. 3,979,715 issued to Hufstedler et al.
After reference signal/vibrator phase lock was ensured, the recorded seismic traces were then correlated with the filtered pilot sweep, or "FPS", utilizing well known seismic processing methods. Assuming that the transfer function of the geophone and the medium could be ignored, the resultant correlated traces were expected to be a zero phase wavelet. Inconsistencies in the resultant correlated wavelets, however, led to recent challenges to this traditional method of seismic processing.
To avoid inconsistencies in the correlated wavelets produced by such methods, W. E. Lerwill, "The Amplitude and Phase Response of a Seismic Vibrator" Geophysical Prospecting, Vol. 29, 1981, pp. 503-528 recommended phase locking between the filtered pilot sweep and the reaction mass acceleration when conducting vibrative exploration. J. J. Sallas, "Seismic Vibrator Control and the Downgoing P-Wave", Geophysics, Vol. 49, June 1984, pp. 732-40, on the other hand, recommended locking the phase of the FPS to the phase of the negative ground force. Still another type of phase locking was recommended by Joseph K. Schrodt, "Techniques for Improving Vibroseis Data", Geophysics, Vol. 52, April 1987, pp. 469-82. There, it was recommended that phase locking the FPS to ground force, along with amplitude control, would provide a more stable downgoing wavelet than phase locking to baseplate or reaction mass acceleration.
Each of the various proposed phase locking methods, however, tended to produce correlated seismic data inconsistent with the correlated seismic data produced by the other proposed phase locking methods. Despite corresponding to the same subsurface reflection points, data acquired using different phase locking techniques generally produced correlated seismic data indicative of differing subsurface characteristics. The information related to the subsurface which could be derived from the acquired seismic varied, therefore, depending on the phase locking technique selected prior to commencing vibrative exploration.